restorer: Fix bootstrap allocation

There are three bugs in this code.

1. self vmas list is released before get-hint is called
2. get-hint code wrongly detects the hole (just bugs, no details)
3. exec hint is mapped without MAP_FIXED, but should

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>